The process of fixation forms the foundation for the preparation of tissue sections. It prevents or arrests autolysis and putrefaction, coagulates and stabilizes soluble and structural proteins, fortifies the tissues against the deleterious effects of subsequent processing and facilitates staining. Current methods of fixation rely on chemical agents, the most widely used being formaldehyde. Although autolysis is known to be retarded by cold and almost inhibited by heating to 60.degree. C. (Drury and Wallington, 1980), heat as a form of tissue fixation has not been exploited in the diagnostic laboratory.
The use of routinely fixed, paraffin-embedded tissue sections for immunohistochemistry staining permits localization of a wide variety of antigens while retaining excellent morphologic detail. However, most chemical fixatives produce denaturation or masking of many antigens and degradation of RNA and DNA. In fact for some antigens, treatment of fixed tissue sections with proteases is required for their demonstration (Brandzaeg, 1982; Taylor, 1986). Furthermore, the introduction of antigen retrieval by heating tissue sections in a microwave oven (Shi et al., 1991) or pressure cooker (Norton et al., 1994; Miller et al., 1995) before immunostaining has been a major breakthrough in improving a result of no or weak immunoreactivity, particularly in suboptimally prepared tissue. However, while the optimal time for fixation varies with the chemical agent employed, this generally takes hours, approximately a day, to accomplish.
Fixative type and fixation time are known to influence 1) the preservation of tissue morphology (Baker, 1959), 2) the preservation of protein antigens for IHC (Williams et al., 1997), and 3) the preservation of nucleic acids for ISH (Weiss and Chen, 1991; Nuovo and Richart, 1989) and PCR (Ben-ezra et al., 1991). It is fortunate that formalin was found to be the best fixative for meeting these three criteria, as this is the fixative most commonly used in routine tissue fixation (Weiss and Chen, 1991; Williams et al., 1997; Nuovo and Richart, 1989).
Increasing the speed and reducing the time of fixation have been investigated using treatments of cold, heat, vacuum, microwave, ultrasound, and microwave combined with ultrasound. Tissues have been microwave irradiated for less than 10 seconds in the presence of chemical cross-linking agents (final solution temperature of 45-70.degree. C.) (Login and Dvorak, 1985; Login et al., 1987). These MW fixation methods used heat to Pasteurize the tissue rather than to fix the tissue. The 10 seconds was not enough time to allow even penetration and complete reaction with the tissues. Also, 70.degree. C. is not hot enough to inhibit all enzymes such as RNases (Sambrook et al., 1989). Furthermore, Azumi and Battifora reported that the improvement seen in antigen preservation in MW fixed tissues was not due to the microwave irradiation per se but rather to the graded alcohol dehydration steps in the tissue processor (Azumi et al. 1990). The exact amount of MW energy received by tissue was very difficult to control (Login and Dvorak, 1985; Azumi et al., 1990). Therapeutic ultrasound (800-880 kHz frequency and 1.4-2 W/cm.sup.2 intensity) did not significantly improve the quality and time of fixation (Drakhli, 1967; Botsman and Bobrova, 1968; Obertyshev, 1987; Rozenberg, 1991) even when combined with MW energy (Shmurun, 1992). Cleaning ultrasound (destructive low frequency 40 kHz) was also used with MW. Disruptions, fissures and cracks of tissues treated for only 3 seconds with ultrasound irradiation (X 45 cycles) were observed when the specimens were examined by light microscopy (Yasuda et al., 1992). Our findings are the same as those reports that low frequency ultrasound exposure can lead to destruction of cell and tissue structure. This indicates that the safety range of low frequency ultrasound is relatively narrow.
In the past decade, molecular pathology has been rapidly developed by using new techniques such as immunohistochemistry (IHC), in situ hybridization (ISH), fluorescent in situ hybridization (FISH), polymerase chain reaction (PCR), reverse transcription (RT)-PCR, and in situ-PCR. The most advanced techniques such as laser capture microdissection (LCM) (Emmert-Buck et al., 1996; Bonner et al., 1997; Fend et al., 1999a; Fend et al., 1999b), cDNA (Schena et al., 1995; DeRisi et al., 1996) and tissue microarrays (Kononen et al. (1998) have been developed for research and diagnosis of molecular pathology. Many genes and signaling pathways controlling cell proliferation, death and differentiation, as well as genomic integrity, have been measured by these techniques in a single experiment, revealing many new, potentially important cancer genes. However, the tissue blocks or sections used for analysis of molecular information of LCM and tissue microarray techniques have not fitted well with the classic method of tissue fixation--formalin fixed paraffin embedded (FFPE) tissue, which has provided the best morphology for pathologists throughout this century (Fend et al., 1999; Goldsworthy et al., 1999).
FFPE tissues have been extensively studied during the last two decades for molecular biology and molecular pathology. There have been many breakthroughs in these areas such as success in isolating the 1918 "Spanish" influenza virus RNA from an 80 year old FFPE tissue block (Taubenferger et al., 1997). However, there are many drawbacks in using FFPE tissue for molecular pathology, such as inconsistency in fixation condition, antigen masking, and RNA/DNA degradation. Even using the advanced microwave-antigen retrieval method (Shi et al., 1991), several CD markers have not worked with FFPE tissues, and the average length of RT-PCR products from FFPE tissues is 200 bp (Fend et al., 1999a; Ben-ezra et al., 1991; Foss et al., 1994; Krafft et al., 1997). All these drawbacks limit the use of LCM and tissue microarray techniques with FFPE tissues (Fend et al., 1999a; Goldsworthy et al., 1999).
Six to eighteen hours are required for routine fixation of surgical tissue specimens. Eight to fourteen hours are required for tissue processing. Additional times are required for embedding, sectioning, staining, and coverslipping of the specimen. A method which simultaneously permits rapid tissue fixation and processing, excellent morphologic detail, antigen preservation, and less RNA/DNA degradation would, therefore, be highly desirable in this molecular pathology era.
For the past three decades, microwave (MW) energy has sometimes been used for rapid tissue fixation (Mayers, 1970; Bernard, 1974; Login, 1978) and tissue processing (Boon et al., 1986) for light and electron microscopy. In the late 60's to early 90's, several Russian groups described a method in which therapeutic ultrasound (US) energy was used for tissue fixation and processing for light microscopy (Drakhli, 1967; Botsman and Bobrova, 1968; Obertyshev, 1987; Rozenberg, 1991) and for electron microscopy (Polonyi et al., 1984; Robb et al., 1991). MW energy combined with US energy was used in conjunction with chemical cross-linking agents to fix and process tissues for light (Shmurun, 1992) and for electron microscopy (Yasuda et al., 1992) at the same time. However, these technologies have not been successfully adopted in clinical diagnostic laboratories and controversial observations of these techniques have been reported (Azumi et al., 1990; Login et al., 1991; Azumi et al., 1991).
This invention relates to a method and apparatus for processing tissue samples or other biological samples for a wide variety of purposes. Tissue samples are analyzed for many purposes using a variety of different assays. Pathologists often use histochemistry or immunocytochemistry for analyzing tissue samples, molecular biologists may perform in situ hybridization or in situ polymerase chain reactions on tissue samples, etc. Often the sample to be analyzed will be frozen or embedded in paraffin and mounted on a microscope slide. A typical immunocytochemistry assay requires a series of many steps. These include: obtaining a tissue sample such as from a biopsy, fixing the tissue in formalin, processing the tissue overnight, embedding the tissue in paraffin, cutting serial sections and mounting on microscope slides and drying. These steps are followed by steps to deparaffinize (treatments in xylene, ethanol and water), and finally the reaction can be performed on the tissue which has been mounted on the slide. Typically a series of solutions including reagents such as antibodies, enzymes, stains, etc., is dropped onto the slide, incubated, and washed off. Finally the sample may be viewed under the microscope. Clearly there are many individual steps involved and each step takes time. The current invention shortens the time for each step to be completed, and therefore shortens the time for the analysis of the tissue sample.
At present, two procedures are generally used in preparing specimens of tissue for microscopic examination. In one procedure a specimen is frozen, cut and mounted on a slide in an elapsed time of about 15 minutes. This so-called "frozen-section" procedure has the advantage of enabling a rapid histological diagnosis to be made from the specimen, and it is frequently employed in situations where a diagnosis is necessary while a patient is on an operating table. The procedure possesses certain disadvantages in that the prepared slide does not possess the uniformity of quality of morphology prepared by other methods. Moreover, it is technically more difficult for serial sections of the same specimen to be examined by this procedure, and extreme caution must be exercised in cutting the specimen in order to ensure a sufficiently thin section and to avoid the possibility of damaging details of the specimen. The most serious objection to using the frozen section procedure is the necessity of preparing all the slides required for special stains and/or consultation and teaching purposes while the tissue is in the initial frozen state. If the tissue is thawed and refrozen for sectioning, it is severely damaged. Thus, when the frozen-section procedure is used in emergency situations, it is customary for another portion of the tissue specimen to be processed in the manner described hereinafter in order to have tissue available for additional sections if further examination becomes necessary.
In the other procedures, a slide of relatively high quality of morphology is produced when a section of the specimen is mounted in a block of paraffin; however, the time required to process a specimen of tissue for mounting in paraffin is on the order of 24-48 hours as compared with the minutes required to process a specimen by the frozen-section procedure. In the preparation of paraffin slides, a specimen of tissue is immersed initially in a fixing agent. The fixed specimen is then immersed in a dehydrating agent, and afterward the specimen is immersed in a clearing agent. Finally, the cleared specimen is immersed in a bath of paraffin which impregnates the specimen and permits it to be sliced into thin sections for subsequent mounting onto slides. Because of the length of time required to prepare specimens by this process, it is customary for hospital laboratories to begin processing the specimens late in the afternoon after surgeons have obtained specimens from their patients. The processing continues through the night, and slides of the specimens are available for microscopic examination the next morning. Although the slides produced according to this procedure are of higher quality of morphology than those produced by the frozen-section technique, the length of time required to process specimens is too great to enable this procedure to be used in situations where time is of the essence.
In the conventional histopathology laboratory, specimens of tissue received from surgery or autopsy are trimmed and preserved in small containers of formaldehyde. The specimens are processed to remove water, and they are mounted in blocks of paraffin which are cut into thin sections. The thin sections are floated on water to enable them to be transferred to slides, and the sections are securely mounted on the slides when they are heated. Thereafter, the paraffin around the mounted sections is removed, and the sections are stained to ready them for microscopic examination.
The ability to obtain rapid results, for example, during an on-going surgery, permits a microscopic examination and diagnosis of a tissue sample and thus, due to this examination, enables suitable surgical steps to be taken during the initial surgery without requiring a follow-up later surgical procedure.
With the foregoing in mind, it is the primary object of the present invention to provide an improved method for preparing specimens of tissue for microscopic examination.
As a further object, the present invention provides an improved method by which tissue specimens of relatively high-quality of morphology can be processed for microscopic examination in a minimum amount of time.
The publications and other materials used herein to illuminate the background of the invention or provide additional details respecting the practice, are incorporated by reference, and for convenience are respectively grouped in the appended List of References.
U.S. Pat. No. 3,961,097 teaches a method of using low frequency ultrasound (50 KHz) to reduce the time to perform biological processes such as fixing tissue and impregnating it with paraffin. This patent teaches placing the sample in a small beaker of reagent to react with the sample and then placing the small beaker into a larger beaker of water which is then irradiated with ultrasound. This method helped to limit damage to the sample from the ultrasound treatment. This varies from the instant invention which places the transducer which produces the ultrasound radiation within one inch of the sample. The instant invention uses ultrasound of a high frequency to minimize damage to the sample.
U.S. Pat. No. 5,089,288 also discusses the use of ultrasound treatment in the processing of tissue samples to impregnate them with paraffin. This method utilized a frequency range of 35-50 KHz. This is a lower frequency than the range of 100 KHz to 50 MHz employed by the instant invention. The higher frequencies of the instant invention result in less biological damage than do the lower frequencies of the prior art '288 patent.
Chen et al. (1984) studied the effect of ultrasound treatment on the rate of an immunoassay performed on a test strip and saw that the reaction was greatly accelerated in the presence of ultrasound. The ultrasound treatment was performed with an ultrasonic cleaner which had a nominal acoustic power output of 50 W at 50 KHz. Tests at various power (watts) were performed by varying the voltage to the sonicator.
Nishimura et al. (1995) teach a method for combining ultrasound treatment with a postfixation step in staining for lipid with osmium tetroxide. The exact specifications as to intensity and frequency of the ultrasound treatment are not disclosed. The ultrasonic treatment was performed using an ultrasonic cleaner. In general, ultrasonic cleaners produce a frequency of 20-50 KHz.
Yasuda et al. (1992) disclose a method for tissue fixation which includes a combination of microwave treatment as well as ultrasonic treatment. Overlapping pulses of a few seconds of microwave and ultrasound were administered for several cycles. The ultrasound generator was set at a dose of 20 W/cm2 and a frequency of 40 KHz and was operated at 25 V. To decrease the occurrence of cavitation which is commonly caused by ultrasound treatment, the experiments were performed at 25 V instead of 100 V, tap water cooled to 0.degree. C. was placed between the cup of the ultrasound generator and the plastic container that contained the tissue blocks and fixative, in order to make the fixative cool and to make three layers (water, plastic and fixative) so that the ultrasound energy would be reduced, and saponin or NP-40 was added to the fixatives to reduce the surface tension of the tissue blocks.
Podkletnova and Alho (1993) utilized ultrasound to increase the rate of performing immunohistochemistry. Samples were placed in plastic tubes which were placed in an ultrasonic bath of cold water (12.degree. C.) and treated with ultrasonic irradiation for 0, 5, 10, 15, 20, 30 or 40 seconds. The sonicator was operated as 220V/50 Hz; 180 W input, 90 W output, and the transducer produced a 40 kHz frequency.
A publication by Polonyi et al. (1984) teaches the use of ultrasound to accelerate glutaraldehyde-osmium fixation of animal tissues. The use of medium intensities with low frequency (20 KHz) gave good results with wet tissue but damage was caused with dehydrated tissue. Consequently the authors adopted a method of using ultrasound during the fixation, washing, postfixation and saturating steps while performing the dehydrating steps without ultrasound.
A publication by Sinisterra (1992) discusses applications of ultrasound to biotechnology. It teaches that high intensity ultrasonic waves break the cells and denature enzymes. Low intensity ultrasonic waves can improve the mass transfer of reagents and products through a boundary layer.